| Compared with traditional optical communication technology,free space optical communication technology has stronger information security and anti-interference ability due to its narrow beam width and good laser directivity.Vortex beams carrying photonic orbital angular momentum(OAM)can improve the information transmission capability and spectral efficiency of existing optical communication technologies.,and Bessel-Gaussian beams with non-diffraction and self-healing properties can reduce the adverse effects of atmospheric turbulence.However,when the vortex beam with orbital angular momentum mode is transmitted in turbulent environment,it will be affected by the signal attenuation caused by the scattering and absorption of atmospheric aerosol and other particles,as well as the orbital angular momentum mode crosstalk,beam spreading and beam drift caused by turbulence,which will inevitably lead to the degradation of the performance of the FSO system.Bessel gaussian localized wave combines the advantages of OAM and BesselGaussian beam and has important research value.Therefore,the study of the wave field characteristics,turbulence effects and orbital angular momentum modal characteristics of Bessel gaussian localized wave in turbulent environment can provide more solutions for the application of vortex beam in free space.Based on the Rytov theory and the generalized Huygens-Fresnel principle,the propagation characteristics of Bessel Gaussian localized wave in atmospheric turbulence and plasma sheathing turbulence layer are studied in this paper,and the methods to improve the transmission quality of OAM in turbulent environment are explored.The main innovative research work of this dissertation includes the following parts:1.The orbital angular momentum modal transmission characteristics of Bessel Gaussian localized wave in turbulent atmosphere under anisotropic conditions from weak to strong turbulence are studied.The OAM propagation model of Bessel Gaussian localized wave in turbulent environment from weak to strong is established by extending the Rytov approximation theory to the modified Rytov theory.The influence of light source characteristics and turbulence effects on the helical spectral characteristics,OAM modal detection probability and channel capacity of Bessel Gaussian localized wave in atmospheric turbulence are analyzed.Compared with traditional Bessel-Gaussian beam,Bessel Gaussian localized wave has better transmission quality and has obvious advantages in anti-turbulence jamming ability.It provides a better choice for improving the performance of OAM free space optical communication system.2.Based on the generalized Huygens-Fresnel principle,the Bessel Gaussian localized wave turbulence effect and OAM transmission characteristics in anisotropic turbulent atmosphere are studied,and the correlation between them is analyzed.The propagation model of Bessel Gaussian localized wave in atmospheric turbulence is established,and the average intensity distribution,beam spreading effect and beam drift effect of the beam in atmospheric turbulence are studied.The beam spreading effect is affected by the inner scale of turbulence,while the beam drift effect is mainly affected by the outer scale refraction of turbulence.The beam spreading and drift effect caused by turbulence can be effectively reduced by using Bessel Gaussian localized wave with non-diffraction characteristics.It provides theoretical support for vortex beam propagation in atmospheric turbulence.3.The propagation characteristics of Bessel Gaussian localized wave in plasma sheath turbulence layer are studied.During the high-speed flight of the hypersonic vehicle,it causes violent collision and extrusion with the nearby air,forming a plasma sheath turbulent layer covering the aircraft,which has a serious impact on the communication problem of the aircraft.Therefore,in this paper,the OAM propagation model of atmospheric turbulence is extended to plasma sheath turbulence medium,and combined with the actual situation of airborne hypersonic vehicle,based on classical turbulence statistical theory and power spectrum model of refractive index fluctuation of plasma sheath turbulence.The intensity distribution,beam spreading effect and OAM mode detection probability of Bessel Gaussian localized wave under different refractive index fluctuation variances and flight conditions are studied.In addition,in order to solve the problem of black barrier,the transmission characteristics of Bessel Gaussian localized wave in the plasma sheath turbulence layer are analyzed.Bessel Gaussian localized wave with low frequency signal can reduce signal attenuation and distortion caused by turbulent scattering,thus effectively reducing the black barrier problem. |
